1. Field of the Invention
The present invention relates to an active matrix display device. In particular, the present invention relates to an active matrix display device including a display element having diode characteristics. The display element having diode characteristics includes, for example, an organic EL (electroluminescence) diode, a light emitting diode, and the like, but is not limited thereto. The display element having diode characteristics refers to the one which exhibits diode characteristics or characteristics close to diode characteristics in the voltage versus current characteristics and thus is changed in the amount of light emission, the transmittance, the reflectance, the color tone, the chroma, and the like to be changed in optical characteristics. Hereinafter, the display element having diode characteristics is also simply referred to as a display element.
2. Description of the Related Art
As a typical example of an electro-optical element having diode characteristics, an organic EL element is given. In addition, an active matrix organic EL display device is known in which organic EL elements are formed in a matrix over a substrate and the organic EL elements are controlled by respective transistors to display an image.
Amorphous silicon, polycrystalline silicon, an oxide semiconductor, or the like is used for a semiconductor layer of a transistor used in an active matrix organic EL display device for the necessity of formation on a large area at a limited temperature range (e.g., see Patent Documents 1 to 3).
In the case of such a transistor including the semiconductor material, variations in the threshold voltage are generally large. In an organic EL display device, the degree of light emission is controlled by the values of currents flowing in the organic EL elements, thereby obtaining a gray scale level. In an active matrix organic EL display device, the values of currents flowing in the organic EL elements are controlled by transistors, and the current value also depends on the threshold voltages of the transistors. Therefore, when the threshold voltage of the transistor is varied, the value of the current flowing in the organic EL element is also varied, so that display lacks uniformity.
In order to suppress a display defect caused by such the variations in the threshold voltage, a technique for correcting the threshold voltage with the use of a plurality of transistors is known (see Patent Documents 2 and 3). Patent Documents 2 and 3 disclose examples in which a threshold voltage correcting circuit is formed using only n-channel transistors, only p-channel transistors, or a combination of n-channel transistors and p-channel transistors.